research papers

Acta Cryst. (2018). B74, 385–393 https://doi.org/10.1107/S2052520618008442 385

Received 3 April 2018

Accepted 7 June 2018

Edited by J. Lipkowski, Polish Academy of

Sciences, Poland

Keywords: HNIW; TNT; HNIW/TNT cocrystal;

high density; green chemical method; energetic

materials.

CCDC reference: 1823458

Supporting information: this article has

supporting information at journals.iucr.org/b

High-density HNIW/TNT cocrystal synthesizedusing a green chemical method

Yan Liu, Chongwei An,* Jin Luo and Jingyu Wang*

School of Environment and Safety Engineering, North University of China, Taiyuan, Shanxi 030051, People’s Republic of

China. *Correspondence e-mail: anchongwei804@sina.cn, wangjingyu804@sina.cn

The main challenge for achieving better energetic materials is to increase their

density. In this paper, cocrystals of HNIW (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-

hexaazaisowurtzitane, often referred to as CL-20) with TNT (2,4,6-trinitro-

toluene) were synthesized using ethanol in a green chemical method. The

cocrystal was formulated as C13H11N15O18 and possesses a higher density

(1.934 g cm�3) than published previously (1.846 g cm�3). This high-density

cocrystal possesses a new structure, which can be substantiated by the different

types of hydrogen bonds. The predominant driving forces that connect HNIW

with TNT in the new cocrystal were studied at ambient conditions using single-

crystal X-ray diffraction, powder X-ray diffraction, Fourier transform–infrared

spectroscopy and Raman spectroscopy. The results reveal that the structure of

the new HNIW/TNT cocrystals consists of three one-dimensional hydrogen-

bonded chains exploiting the familiar HNIW–TNT multi-component supramo-

lecular structure, in which two hydrogen-bonded chains are between —NO2

(HNIW) and —CH (TNT), and one hydrogen-bonded chain is between —CH

(HNIW) and —NO2 (TNT). The changes to the electron binding energy and

type of element in the new cocrystal were traced using X-ray photoelectron

spectroscopy. Meanwhile, the physicochemical characteristics alter after

cocrystallization due to the hydrogen bonding. It was found that the new

HNIW/TNT cocrystal is more thermodynamically stable than HNIW. Thermo-

dynamic aspects of new cocrystal decomposition are investigated in order to

explain this observation. The detonation velocity of new HNIW/TNT cocrystals

is 8631 m s�1, close to that of HNIW, whereas the mechanical sensitivity is lower

than HNIW.

1. Introduction

Currently, development of new energetic materials with

insensitive high explosives is of great interest for applications

in civil constructions, explosives, rocket propellants and airbag

inflators (Liu et al., 2015). For this reason, cocrystallization has

attracted considerable attention due to its ability to modify

and optimize the physicochemical properties of energetic

materials (Bennion et al., 2015; Wu et al., 2015). A cocrystal

can be defined as multicomponent crystals of neutral mole-

cular species in a specified ratio and is generally stabilized by

hydrogen bonding, �–� stacking, p–� stacking, halogen bonds

and van der Waals forces, instead of the common chemical

bond (Bond, 2007; Stahly, 2009; Landenberger & Matzger,

2012). This new crystal engineering has been successfully

applied broadly in the fields of explosives, propellants and

pyrotechnics, and is recognized as a powerful tool for altering

ISSN 2052-5206

properties of existing energetic materials to create less-sensi-

tive explosives (Shi et al., 2016; Sun et al., 2018).

HNIW, a non-aromatic cyclic nitramine, is a typical high-

energy-density explosive. Higher specific impulse, ballistics,

oxygen balance, detonation velocity and detonation pressure

make it a better energetic material than other nitramines.

However, HNIW displays sensitivity to high impact, friction,

shock waves and electric sparks which makes it seriously

limited for civilian application and military use (Sun et al.,

2018; Turcotte et al., 2005; Liu et al., 2016). TNT possesses low

density, oxygen balance, detonation velocity and detonation

pressure and is defined as an insensitive energetic material

(Wilson et al., 1990). Therefore, HNIW/TNT cocrystals have

been synthesized via a cocrystal strategy. HNIW/TNT

cocrystals improve the sensitivity of the HNIW but the energy

output is better still. The chemical structures of the cocrystal

coformers, HNIW and TNT, are shown in Scheme (1).

In recent years, solution crystallization methods have been

used to obtain cocrystals (Xu et al., 2015; Bennion et al., 2017;

Yang et al., 2013). A suitable organic solvent was used to

directly dissolve the whole cocrystal coformers, but these

organic solvents are commonly toxic (Lin et al., 2016; Gao et

al., 2017; Ghosh et al., 2012). An obvious disadvantage of this

method is that the application of large amounts of toxic

organic solvents and their removal by evaporation will result

in health hazards and environmental pollution. To address

these problems, we employed a green chemical method to

synthesize a new HNIW/TNT cocrystal. This method

emphasizes that the organic solvent used is greener, safer and

more environmentally benign.

Hence, in this current work we replace the toxic organic

solvent (ethyl acetate) with a less-toxic solvent, ethanol, to

reduce environmental destruction and damage to health.

Because HNIW (also known as CL-20) is slightly soluble

(0.503 g per 100 ml at 25�C) in ethanol (Holtz et al., 1994),

grinding was introduced to assist dissolution. Due to a faster

and cleaner dynamic kinetic resolution, HNIW is dissolved in

ethanol (at least to some extent) with the assistance of

grinding. Moreover, compared with the ethyl acetate solvent

(40.6 g CL-20 per 100 ml ethyl acetate at 25�C; Holtz et al.,

1994), the crystals are easier to grow from a poor solvent

(ethanol) during the slow evaporation from solution (Xiao et

al., 2014). So, this green chemical approach using the less-toxic

poor solvent ethanol is significantly different from the

commonly used slow evaporation from solution and is also a

better choice for growing the new HNIW/TNT cocrystal. In

this paper the elemental change, crystal structure, explosive

properties and the safety properties of the new cocrystal were

investigated in detail.

2. Experimental

2.1. Materials

HNIW and TNT were provided by Modern Chemistry

Research Institute of China. Ethanol was purchased from

Tianjin Hengxing Chemical Co. Ltd of China.

2.2. Preparation of HNIW/TNT cocrystals by green chemicalmethods

HNIW (4.38 g, 10 mmol) and TNT (2.27 g, 10 mmol) were

introduced into a grinding jar with a large quantity (133 g) of

zirconia grinding balls (diameter 0.2 mm), followed by 33.3 ml

of ethanol. The jar was then quickly closed to prevent vola-

tilization of ethanol. Grinding was performed for 2 h at 3000

rpm on a HWNSM-0.3L grinder (Shanghai HWEI Mechanical

& Electrical Equipment Co., Ltd) at 220 V and 50 Hz. The

experiment was performed at room temperature. Single

crystals of the new HNIW/TNT cocrystal were obtained by

slow evaporation of ethanol filtrate containing the material

prepared by grinding.

In this experiment, ethanol was used as an organic solvent,

which is less toxic and is a better choice for our original aim,

green chemistry. Furthermore, a grinder was used to assist

dissolution of HNIW. It highlights a faster and cleaner

dynamic kinetic resolution and grind technology as a scalable,

sustainable and environment friendly strategy. Application of

the grind techniques can reduce handling time, decrease

reaction temperature, circumvent toxic solvents and reactant,

minimize production costs, improve reaction rates and

increase reaction yield. Therefore, our experiment is condu-

cive to reduce environment destruction and damage to health.

It further executes the green behavior.

A general synthesis schematic of the green chemical process

is shown in Fig. 1.

2.3. Characterizations methods

2.3.1. Scanning electron microscopy. The particle

morphology, size distribution, crystal structure and composi-

tion analysis of the new HNIW/TNT cocrystals and the co-

formers were examined using a MIRA3 LMH scanning elec-

tron microscope (Tescan, Czech Republic).

2.3.2. Single-crystal X-ray diffraction (SXRD). SXRD data

of the new HNIW/TNT cocrystals with suitable quality were

collected using a Bruker Smart Apex-II X-ray single-crystal

diffractometer. The instrument was equipped with graphite-

monochromated Mo K� radiation (� = 0.71073 A) at 50 kV

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386 Yan Liu et al. � High-density HNIW/TNT cocrystal Acta Cryst. (2018). B74, 385–393

Figure 1Schematic illustration of the set-up for the green chemical methodsynthesis of the new cocrystal.

and 30 mA. A total of 21696 reflections was collected (�11 �

h� 11,�19� k� 22,�29� l� 28), of which there were 4020

unique reflections (Rint = 0.0518). The crystal structure was

refined by full-matrix least-squares methods, and absorption

correction was semi-empirical from equivalents. All hydrogen

atoms were refined anisotropically and fixed at 0.93 A. Non-

hydrogen atoms were refined using anisotropic thermal

parameters.

2.3.3. Powder X-ray diffraction (PXRD). PXRD data of the

new HNIW/TNT cocrystals and the raw materials on lightly

ground bulk materials were collected with a DX-2700 powder

X-ray diffractometer (Dandong Haoyuan Corporation,

Liaoning, China) operating at a tube voltage of 40 kV and a

current of 30 mA using Cu K� radiation at � = 1.5418 A. The

patterns were scanned in the range 5 � � � 50� with a 0.03�

step size. Additionally, PXRD patterns were analyzed using

Jade 9 (Materials Data Inc., Livermore, CA, USA) and PDF-2

(International Centre for Diffraction Data, 2009) software.

2.3.4. Fourier transform infrared (FT–IR) spectroscopy.

FT–IR was performed using a Spectrum 100 infrared spec-

trometer (Perkin-Elmer, Inc.). Samples were prepared as KBr

disks and spectra were recorded over the range 400–

4000 cm�1 with a resolution of 4 cm�1. Each spectrum was the

average of 32 individual scans.

2.3.5. Raman spectroscopy. The Raman spectra of the new

HNIW/TNT cocrystals and the raw materials were recorded

using a LabRAM HR800 laser confocal micro-Raman spec-

trometer (Horiba Jobin Yvon Corporation, France).

2.3.6. X-ray photoelectron spectroscopy. The element of

the samples was traced by a Thermo ESCALAB 250XI X-ray

photoelectron spectrometer (Thermo Fisher Scientific,

America).

2.3.7. Differential scanning calorimetry. Thermal behavior

of the samples was measured using a differential scanning

calorimeter (Setaram DSC-131, Setaram Corporation, France)

by heating 0.7 mg of HNIW/TNT cocrystals in a hermetically

sealed aluminium crucible with a pinhole in the lid at a heating

rate of 10�C min�1. The variable-temperature DSC curves for

HNIW, new HNIW/TNT cocrystals and TNT were recorded

from 25 to 325�C under nitrogen flow (30 ml min�1).

2.3.8. Detonation velocity. Detonation velocity is an

important parameter to evaluate the energetic performance of

the HNIW/TNT cocrystals, which was tested by a timing

method. A schematic of the detonation velocity measurement

is given in Fig. 2. A full description of the measurement

technique is given in the supporting information.

2.3.9. Mechanical sensitivity test. The impact sensitivity

was determined with an ERL Type 12 drop-hammer apparatus

according to GJB-772A-97 standard method 601.2 (National

Military Standard of China, 1997). The experimental set-up

for measuring impact sensitivity is shown in Fig. 3. The testing

conditions are: drop weight of 2.500 � 0.002 kg; sample mass

of 35 � 1 mg; ambient temperature of 20�C and relative

humidity of 30%. The samples was placed on the hammer

anvil and hit by a drop hammer. When the drop hammer

dropped, the special height (H50) represented a critical drop

height with 50% explosion probability. The test was carried

out 30 times to characterize the impact sensitivity of the

samples. A full description of the impact sensitivity experi-

ment is given in the supporting information.

The friction sensitivity was tested on a WM-type friction

sensitivity apparatus according to GJB-772 A-97 standard

method 602.1 (National Military Standard of China, 1997).

The testing conditions are as follows: pendulum weight of

1.50 � 0.01 kg, sample mass of 20 � 1 mg, relative pressure of

4.9 MPa, swaying angle of 90�, ambient temperature of 20�C

and relative humidity of 30%. The friction sensitivity of each

test sample was expressed by explosion probability (P). The

test was carried out continuously 30 times to characterize the

friction sensitivity and explosion probability of the samples.

The experimental set-up for measuring the friction sensi-

tivity is shown in Fig. 4. A full description of the friction

sensitivity experiment is given in the supporting information.

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Acta Cryst. (2018). B74, 385–393 Yan Liu et al. � High-density HNIW/TNT cocrystal 387

Figure 4Experimental set-up for measuring friction sensitivity.

Figure 3Experimental set-up for measuring impact sensitivity.

Figure 2Schematic of detonation velocity measurement using a timing method.

3. Results and discussion

3.1. Scanning electron microscopy (SEM)

Crystal quality (crystal shape, crystal size, crystal surface

and crystal defects) plays an important role during storage,

transport and use of explosives. These physicochemical para-

meters may affect the detonation initiation spots of the

explosive (Shen et al., 2011). The morphological differences of

HNIW, TNT and the synthesized HNIW/TNT cocrystal are

illustrated in Fig. 5.

HNIW of about average particle size 100 mm possesses a

hippocrepiform-type crystal morphology with sharp crystal

edges; nevertheless, many crystal defects such as dislocations

and grain boundaries were observed to exist on the crystal

round (Fig. 5a). TNT is a irregular flake (Fig. 5b) with a coarse

surface and lots of particle deposits appear on the surface. The

dimensions of the flake range from 2 to 3 mm. In contrast, the

surfaces of the new HNIW/TNT cocrystals are smooth, their

shapes are regular and their form is hexahedral, as shown in

Fig. 5(c). The particle size of the HNIW/TNT cocrystals is

about 1 mm, which is clearly large than for HNIW. Their

different morphologies are most likely to affect their physi-

cochemical properties (Yang et al., 2014). The above results

unambiguously indicate that the new HNIW/TNT cocrystal

possesses a unique crystal shape and crystal size that is unlike

HNIW and TNT. The cocrystal strategy not only drastically

alters the crystal shape but also the crystal size.

3.2. Single-crystal X-ray diffraction (SXRD)

The structure of the new HNIW/TNT cocrystals was

determined by SXRD. The packing arrangement is presented

in Fig. 6. Detailed crystallographic data and structure refine-

ment parameters for the new HNIW/TNT cocrystals and the

previous reported HNIW/TNT cocrystal (Yang et al., 2013) are

summarized in Table 1. The hydrogen bond geometry between

the HNIW and TNT in the new cocrystal structure and the

previously reported cocrystal (Yang et al., 2013) are given in

Table 2.

The new cocrystal structure comprises two independent

molecules (HNIW and TNT) and it is an asymmetric unit

Fig. 6(a). In the new cocrystal, three cooperative hydrogen

bonds link adjacent HNIW and TNT molecules, forming one-

dimensional chains along the c-axis direction [Fig. 6(b) and

Table 2). The one-dimensional chained structure extends

infinitely in two directions, forming a two-dimensional

network. Then, the two-dimensional networked structure

stacks continuously, generating a three-dimensional arrange-

ment along the c-axis direction (Fig. 6c). The three-dimen-

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388 Yan Liu et al. � High-density HNIW/TNT cocrystal Acta Cryst. (2018). B74, 385–393

Figure 5SEM micrographs of crystals of (a) raw HNIW, (b) TNT and (c) the newHNIW/TNT cocrystal.

Table 1Crystal data and structure refinement parameters.

New cocrystalstructure

Cocrystal reported byYang et al. (2013)

Chemical formula C13H11N15O18 C13H11N15O18

Formula weight 665.37 665.37Color Light yellow ColorlessMorphology Hexahedron PrismTemperature (K) 296 293.15Crystal system Orthorhombic OrthorhombicSpace group Pbca Pbcaa (A) 9.6268 (12) 9.7352 (2)b (A) 19.292 (2) 19.9121 (6)c (A) 24.606 (3) 24.6955 (6)Unit-cell volume (A3) 4569.8 (10) 4787.20 (10)Z 8 8Calculated density (g cm�3) 1.934 1.846Absorption coefficient (mm�1) 0.181 -F(000) 2704 -�min, �max (�) 2.11, 25.00 6.08, 52.74No. of reflections collected 21696 12659No. of independent reflections 4020 4892No. of data, restraints, parameters 4020, 0, 416 -Goodness-of-fit 1.083 1.023Final R1, wR2 indices [I > 2�(I)] 0.0372, 0.0990 0.0489, 0.1166R1, wR2 indices (all data) 0.0429, 0.1022 -Extinction coefficient 0.0011 (2) -CCDC No. 1823458 -

Figure 6The new HNIW/TNT cocrystal: (a) ORTEP view, (b) view down the caxis of the one-dimensional layered structure and (c) view down the c axisof the three-dimensional arrangement.

sional structure is a sine wave pattern and propagates in the

crystal.

Table 2 shows that three hydrogen bonds exist between

molecules of HNIW and TNT by which the new HNIW/TNT

cocrystals assemble into a two-component cocrystal. However,

only two hydrogen bonds (C—H8� � �O3 and C—H5� � �O13)

exist in the previous cocrystal. This interaction of hydrogen

bonds is assumed to be reason for the formation of the new

cocrystals. It is commonly noted in other energetic cocrystals

and may be an important consideration in the design of future

materials (Bolton et al., 2012). Other differences from the

previous cocrystal are hydrogen bond lengths and bond

angles. A different crystal structure shows different macro-

scopic morphology and different properties. Thus, the color,

shape and crystal density are different, which can be seen from

Table 1.

In the new structure, the crystal is light yellow and a

hexahedron. However, in the structure reported previously,

the crystal is colorless and a prism. Aside from color and

shape, the density is also significantly improved to that

published previously. In the new structure, the density is

1.934 g cm�3, which is an increase of �5% on the previously

reported density (1.864 g cm�3). This result further illustrates

that different crystal structure does make macroscopic

morphologies and properties.

Particularly, in the field of energetic materials, HNIW

(CL-20) possesses a maximum crystal density of 2.035 g cm�3

(only for "-CL-20) and aside from a few instances, the others

are less than 2 g cm�3 (Zhang et al., 2017). So, making the

density value of energetic materials more than 2 g cm�3 is

a huge challenge due to its inherent nature. Analogously,

a great challenge also appears at more than 1.9 g cm�3.

Therefore, 1.934 g cm�3 in the new HNIW/TNT cocrystal is a

great breakthrough in energetic materials.

The new crystal structure also shows other different prop-

erties such as DSC, detonation velocity and mechanical

sensitivity. This has been proven by more stable thermal

behavior, higher detonation velocity and safer mechanical

sensitivity. Aside from these three properties reported by

Yang et al. (2013), we report additional studies (PXRD, FT–

IR, Raman spectroscopy and XPS).

3.3. Powder X-ray diffraction (PXRD)

X-ray diffraction (PXRD) measurements were carried out

to identify whether a new polymorphism in new HNIW/TNT

cocrystals (Xiong et al., 2017). PXRD analysis of the HNIW,

new cocrystals, and TNT gave a diffractogram in Fig. 7.

The pattern reveals that the new HNIW/TNT cocrystal

exhibits unique peaks at 2� ’ 15.97�, 26.04�, 28.09� and 30.57�,

which is evidently different from those of the co-formers.

Simultaneously, in the 2� (�) ranges 21.69–23.14, 25.27–26.05

and 28.09–30.56, the diffraction intensity of the new HNIW/

TNT cocrystals is much stronger than the coformers. It can

also be observed that the main sharp peaks for the HNIW at 2�’ 27.84�, and for the TNT at 2� ’ 33.93� and 42.83� disappear

in the pattern of the new cocrystal. The weak or disappeared

diffraction intensity of the original diffraction peaks and the

strong or occurred diffraction intensity of the new diffraction

peaks clearly demonstrate that the synthesized material by a

green chemical method completely converts to the new

HNIW/TNT cocrystals form and is not a product of crystal

transformation or a simply physical accumulation by HNIW

and TNT particles at a macroscopic scale, but they have

interacted to change inherent structure and component

forming a new crystal. Good agreement with the results of

single crystal showed that a new HNIW/TNT cocrystals

structure was obtained by green chemically synthesized.

3.4. Fourier transform infrared (FT–IR) spectroscopy

FT–IR analysis was employed to identify the molecular

structure of the HNIW, new HNIW/TNT cocrystals and TNT,

and the results are displayed in Fig. 8. In the IR spectrum of

the new HNIW/TNT cocrystals, the absorption peaks located

at 3053 cm�1 and 1518 cm�1 correspond to the stretching

vibrations of C—H and N—NO2, respectively. This means that

in the cocrystallization process, the –CH (in TNT) reacted

with N—NO2 (in HNIW) to form a hydrogen bond, and this

observation agrees with the SXRD data. The peak at

1176 cm�1 reflects the stretching vibrations of C—N. Peaks at

994 cm�1 and 879 cm�1 reflect the deformation vibrations of

ring vibrations of TNT and the –NO2 (in HNIW), respectively.

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Acta Cryst. (2018). B74, 385–393 Yan Liu et al. � High-density HNIW/TNT cocrystal 389

Table 2Hydrogen bond lengths and bond angles between HNIW and TNT in thenew structure and the previously reported structure.

C—H� � �O H� � �O (A) C� � �O (A) C—H� � �O (�)

New structureC2—H2A� � �O10i 2.51 3.427 (2) 168C4—H4A� � �O14ii 2.44 3.316 (2) 157C13—H13A� � �O1iii 2.37 3.173 (2) 138Previously reported structure†C—H8� � �O3 2.31 – 146C—H5� � �O13 2.62 – 161

† Yang et al. (2013). Symmetry codes: (i) 1 � x, �y, 1 � z; (ii) x, 12� y, � 1

2� z; (iii)12� x, �y, 1

2þ z.

Figure 7PXRD diffractogram of (a) HNIW, (b) new HNIW/TNT cocrystals and(c) TNT.

Overall, the change of the IR absorption peaks in the new

cocrystal is due to the formation of the hydrogen bond. The

results indicate that the introduction of a donor–acceptor

interaction between HNIW and TNT represents a reliable

supramolecular synthon for the formation of new HNIW/TNT

cocrystals, which has a profound influence on the packing of

the molecules in the cocrystals of the energetic material

(Landenberger & Matzger, 2010).

3.5. Raman spectroscopy

To explore the reason of cocrystal formation, Raman

analysis was carried out. The Raman spectra and the assign-

ment of the major vibrational bands for the HNIW, new

HNIW/TNT cocrystal and TNT in the region 200–3500 cm�1

are shown in Fig. 9 and Table 3, respectively. The results

indicated that in the new HNIW/TNT cocrystal, a large

number of characteristic peaks shifted markedly. From Fig. 9,

we can see that compared with HNIW and TNT, peaks in the

new cocrystal are obvious and sharp, mainly over the region

750–1700 cm�1. In particular, the C—H stretching vibration in

HNIW at 3049.6, 3033.0 and 3024.7 cm�1 shifted to 3127.8,

3044.1 and 3037.2 cm�1 in the new HNIW/TNT cocrystal. The

NO2 asymmetric stretching in TNT at 1628.6 cm�1 shifted to

1623.8 cm�1 in the new HNIW/TNT cocrystal. The shift of

these characteristic vibrational bands are due to hydrogen

bond C13—H13A� � �O1 between HNIW and TNT. Further-

more, the C—H stretching vibration in TNT at 1360.9 cm�1

shifted to 1362.4, 1347.1 and 1340.1 cm�1 in new HNIW/TNT

cocrystal. The NO2 asymmetric stretching in HNIW at 1630.7,

1623.8, 1609.9 and 1596.1 cm�1 shifted to 1672.2 cm�1 in the

new HNIW/TNT cocrystal. This phenomenon can be attrib-

uted to hydrogen bonds C2—H2A� � �O10 and C4—

H4A� � �O14 between HNIW and TNT. These results are

consistent with the SXRD analysis.

3.6. X-ray photoelectron spectroscopy (XPS)

To trace the elemental change of HNIW and TNT and the

new cocrystal, XPS analysis was employed, and the XPS

spectra are shown in Figs. 10, 11, 12 and 13.

The survery spectra illustrated in Fig. 10 show that three

elements (N 1s, O 1s and C 1s) were detected in each sample.

By analysis of the N 1s spectra of three samples in Fig. 11, two

peaks appeared in HNIW with binding energy of 401.16 and

406.47 eV, respectively, which arises from C—N—C and N—

NO2, respectively. Only one peak occurred in TNT with

binding energy of 403.41 eV, attributing to C—NO2. However,

there are three peaks in the new HNIW/TNT cocrystal with

binding energies of 400.56, 404.43 and 407.29 eV. It can be

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390 Yan Liu et al. � High-density HNIW/TNT cocrystal Acta Cryst. (2018). B74, 385–393

Figure 9Raman spectra of HNIW, new HNIW/TNT cocrystal and TNT.

Figure 8IR spectra of the samples.

Table 3Assignments of the major bonds from 200 to 3500 cm�1 of the Ramanspectra of samples.

AssignmentHNIW(cm�1)

NewHNIW/TNT(cm�1)

TNT(cm�1)

C—H stretching 3049.6 3127.8 3107.03033.0 3044.13024.7 3037.2

CH3 stretching 2965.1 2967.9N—N stretching (N—NO2) 1672.2NO2 asymmetric

stretching1630.71623.81609.91596.11575.3

C—N stretching (C—NO2) 1623.8 1628.6C C stretching 1554.6 1547.7

1540.8C—H stretching 1381.6 1362.4 1360.9

1305.5 1347.11340.1

C—C stretching 1249.5 1201.1 1214.91235.7 1104.21221.8

C—H deformation 1097.31035.0993.5

Ring stretching 965.8 945.1938.2924.3

Ring deformation 862.1 840.6819.9

C—C stretching 826.8 822.5806.0 799.1785.3757.7

Cage deformation 293.4 369.5231.1 314.2286.5217.3

seen that significant shift was observed from 401.16 to

400.56 eV and 406.47 to 407.29 eV of the two peaks in HNIW.

Meanwhile, a shift from 403.41 to 404.43 eV of one peak was

observed in TNT. High-resolution XPS shows one peak of O

1s centered at 533.49 eV in new HNIW/TNT cocrystal. This

binding energy was a obvious shift, comparing with 534.32 eV

in HNIW and 532.04 eV in TNT. Due to an analogous

chemical environment for —NO2, the shapes of all the O 1s

peaks are similar. Fig. 13 shows the C 1s spectra of HNIW, new

cocrystal and TNT, in which the characteristic peaks are at the

binding energies of 289.69, 288.39 and 286.46 eV, respectively.

The reasons for binding energy are due to C—N in HNIW and

C—C or C—NO2 in TNT. However, the peak shifts markedly

in the new HNIW/TNT cocrystal. In the three high-resolution

XPS spectra we can clearly see that due to the hydrogen

bonds, the position and bind energy of peaks in new cocrystal

are obvious different from HNIW and TNT. Furthermore, no

additional signal was detected in the spectrum, implying that

no other functional groups are attached and the samples are

very pure after green chemical synthesis.

3.7. Differential scanning calorimetry

The thermal behavior of HNIW, new HNIW/TNT cocrystals

and TNT was examined by differential scanning calorimetry

(DSC). DSC thermograms of HNIW, new HNIW/TNT

cocrystals and TNT are shown in Fig. 14 and the experimental

results are presented in Table 4.

HNIW gives a strong exothermic peak at 248.91�C attrib-

uted to the decomposing of the crystal (Yang et al., 2012; Foltz

et al., 1994). TNT has an endothermic peak at 80.87�C,

corresponding to its melting point (Shamim et al., 2015).

Furthermore, a unique exothermic peak is observed at

319.25�C. However, the endothermic peak of the new HNIW/

TNT cocrystals locates at 142.59�C; simultaneously, there are

two obvious exothermic decomposition peaks. The first

presents at 221.68�C and the second appear at 249.97�C, which

correspond to low-temperature decomposition and high-

temperature decomposition, respectively (Cheng et al., 2016).

Consequently, it is notable that some significant changes are

given in the thermal properties of new HNIW/TNT cocrystals

and a distinct thermal behavior is presented in the endo-

thermic decomposition and exothermic decomposition.

Furthermore, the stability of the new cocrystal is improved

due to the hydrogen bonding in the structure. These results

demonstrate that such a new cocrystal structure via a green

chemical process provides an efficient route for excellent

thermal property and stability.

3.8. Detonation velocity

To evaluate explosive performance, the detonation velo-

cities of HNIW, new HNIW/TNT cocrystals, the previously

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Acta Cryst. (2018). B74, 385–393 Yan Liu et al. � High-density HNIW/TNT cocrystal 391

Figure 10XPS survey spectra of HNIW, new HNIW/TNT cocrystal and TNT.

Figure 11High-resolution detail of N 1s peak of HNIW, new HNIW/TNT cocrystaland TNT by XPS.

Figure 12High-resolution detail of O 1s peak of HNIW, new HNIW/TNT cocrystal,and TNT by XPS.

Figure 13High-resolution detail of C 1s peak of HNIW, new HNIW/TNT cocrystaland TNT by XPS.

reported cocrystal and TNT were measured using a timing

method. The detonation velocity (D) is calculated as L/t,

where L is the height of the explosive cylinders and t is the

travel time of the detonation wave between two probes.

Because the values of t and L exhibit some difference during

the six successful tests, the final apparent detonation velocity

D of each sample is expressed as the average value, in this case

from six successful tests.

The D values for HNIW, new HNIW/TNT cocrystal,

previously reported cocrystal and TNTare presented in Fig. 15.

The detonation velocity of the new cocrystal (8631 m s�1) in

our manuscript is higher than for the previously reported

cocrystal (8426 m s�1; Yang et al., 2013) and this result is also

higher than for TNT (6854 m s�1). The reason can be

explained as follows. In the field of energetic materials, the

density value seriously affects detonation velocity. Increased

crystallographic density can decrease the volume and the

number of cavities in the crystal structure. Therefore, when

the volume of explosive cylinders remains constant, high

density leads to high mass. Under the same conditions, the

higher the mass of the explosive, the greater the energy ouput

and the higher the detonation velocity, i.e. under the same

conditions, a higher density energetic material shows a

promising detonation velocity (Zhang et al., 2017). Due to the

higher density (1.934 g cm�3 versus 1.846 g cm�3, see Table 1),

a higher detonation velocity has been measured in the new

cocrystal.

Although the HNIW, with a detonation velocity of

9329 m s�1, is more powerful than new HNIW/TNT cocrystals,

it is obvious that HNIW possesses high sensitivity and that

occurs for phase transition which is accompanied by the

increase in temperature. For example, all polymorphs of

HNIW convert to the �-form once heated above 150�C (Foltz

et al., 1994). In contrast, new HNIW/TNT cocrystals are more

thermodynamically stable than HNIW, which is in accordance

with data in Fig. 14.

The results show that the introduction of the green chemical

methods to cocrystallize HNIW with TNT can improve its

detonation property by the interaction of hydrogen bonds.

This is helpful to increase the detonation properties of ener-

getic materials.

3.9. Mechanical sensitivity test

The safety performance of HNIW, new HNIW/TNT

cocrystals, and TNT is investigated by estimation the impact

and friction sensitivities for stimuli. The results are listed in

Table 5.

The special height (H50) and explosion probability (P) of

the new cocrystals are 43 � 0.02 cm and 49%, respectively,

which indicates that the H50 value increases by 231% and P

has an obvious decrease of 51% for the new HNIW/TNT

cocrystal reflecting a evidently lower mechanical sensitivity

than HNIW. The reason is that the HNIW and TNT molecules

are packed compactly and uniformly through intermolecular

interactions of hydrogen bonds and other directional inter-

actions during cocrystallization (Wei et al., 2015). This packing

stabilizes the structure and efficiently reduces the volume and

number of voids in the crystal structure, therefore leading to a

reduction in the probability of formation hot spots under

impact and friction stimuli. The explosion of an explosive is

due to the formation of hot spots under extrinsic stimulation

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392 Yan Liu et al. � High-density HNIW/TNT cocrystal Acta Cryst. (2018). B74, 385–393

Table 4Summarized data from DSC thermograms of HNIW, new HNIW/TNTcocrystals and TNT.

SpecimenMelting point(�C)

Endothermicdecomposition(�C)

Exothermicdecomposition(�C)

HNIW - - 248.91New HNIW/TNT - 142.59 221.68, 249.97TNT 80.87 - 319.25

Figure 14DSC thermograms of HNIW, new HNIW/TNT cocrystals and TNTheating rate of 10�C min�1.

Table 5Results of the mechanical sensitivity test for HNIW, new HNIW/TNTcocrystals and TNT.

Samples

Impact sensitivity Friction sensitivity

H50 (cm) Drop (%) P (%) Drop (%)

Raw HNIW 13 - 100 -New cocrystals 43 "231 49 #51Raw TNT 95 - 38 -

Figure 15The D values of HNIW, cocrystals and TNT as measured by a timingmethod. The error bars represent the standard deviation for the averagevalue obtained from six effective tests.

(Field, 1992; Wang et al., 2016). So, the new stable cocrystal

structure can dramatically improve the mechanical sensitivity.

The results illustrate that the new cocrystal is more insen-

sitive to mechanical stimuli due to its unique crystal structure

formed by cocrystallization. When the new cocrystal under-

goes an external stimulation action, the close and uniform

molecule structure acts as a buffer system to dissipate the

impact and friction energy. It demonstrates that an obvious

desensitization effect has been achieved for the prepared new

HNIW/TNT cocrystals via a green chemical method.

4. Conclusion

Herein, we report a new HNIW/TNT cocrystal with higher

density and better properties than previous reported (Yang et

al., 2013) for the cocrystal, which was prepared via a green

chemical method. The results indicate microscopically and

macroscopically that the cocrystal is a new structure due to the

new set of hydrogen bonds between the HNIW and TNT. Our

investigations showed that this new cocrystal structure

possesses preferable crystal quality, more stable thermal

property and lower mechanical sensitivity. In particular, the

crystal density and detonation velocity are excellent compared

with those of the cocrystal reported by Yang et al. (2013). This

synthesis is environmentally benign, scalable and a sustainable

strategy.

Moreover, these results also make it clear that with the

assistance of grinding, a poor solvent can be employed in a

solvent evaporation process. This technological innovation

indicates the transformative potential of the solvent

evaporation method in the crystal-growing lexicon and should

stimulate more research to further its systematic development.

This study should also encourage further studies into the

design and synthesis of more functionally interesting cocrystal

materials.

Funding information

The following funding is acknowledged: Advantage Disci-

plines Climbing Plan of Shanxi Province.

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